This invention pertains to stablized aqueous zirconium and titanium crosslinking compositions for use with solvatable polysaccharides to form stabilized crosslinked gels. This invention also pertains to processes for using such gels as fracture fluids for fracturing subterranean formations.
The present invention includes a process for fracturing a subterranean formation penetrated by a wellbore which comprises the steps of:
(a) substantially continuously injecting a crosslinking amount of the composition of the invention into a flowing stream of an aqueous solvatable polysaccharide to thereby form a flowing crosslinkable fracturing fluid,
(b) introducing said flowing crosslinkable fracturing fluid into the wellbore at a flow rate and pressure sufficient to permit said crosslinkable fracturing fluid to gel and increase in viscosity while it is passing through the borehole, and,
(c) introducing said aqueous gelled fracturing fluid into said formation at a flow rate and pressure at least sufficient to create, reopen and/or extend a fracture in said formation.
Hydraulic fracturing is a term that has been applied to a variety of techniques used to stimulate the production of oil, gas and other fluids from subterranean formations. In hydraulic fracturing, a suitable fracturing fluid is introduced into a subterranean formation by way of a wellbore under conditions of flow rate and pressure which are at least sufficient to create and/or extend a fracture into a desired part of the formation. The fracturing fluid normally carries with it a proppant (e.g. sand, bauxite, etc.) which is forced into the fracture itself and keeps the broken formation from closing down upon itself once the pressure is released.
Various fluids under pressure have been used in hydraulic fracturing. Most of the fracturing fluids used today are aqueous-based liquids which have been either gelled or foamed.
Aqueous gels are usually prepared by blending a polymeric gelling agent with an aqueous medium. Most frequently, the polymeric gelling agent of choice is a solvatable polysaccharide. These solvatable polysaccharides form a known class of compounds which include a variety of natural gums as well as certain cellulosic derivatives which have been rendered hydratable by virtue of hydrophilic substituents chemically attached to the polymer backbone. The solvatable polysaccharides thereof include galactomannan gums, glucomannan gums and cellulose derivatives. Examples of such polymers include guar, carboxyalkyl guar, hydroxyalkyl guar, and carboxyalkyl hydroxyalkyl guar, galactomannan gums, glucomannan gums, xanthan gums, and the like.
The solvatable polysaccharides have a remarkable capacity to thicken aqueous liquids. Even small amounts are suficient to increase the viscosity of such aqueous liquids from 10 to 100 times or more. In many instances, the thickened aqueous liquid has sufficient viscosity to carry the proppant during the course of the fracturing process and represents a satisfactory fracturing fluid. In other instances, it is necessary to crosslink the polysaccharide in order to form a gel having sufficient strength and viscosity to carry out the proppant. A variety of crosslinkers have been developed to achieve this result.
The borate ion has been used extensively as a crosslinking agent for hydrated guar gums and other galactomannans to form aqueous gels used in fracturing and other areas. For example, Kern described a crosslinked system in U.S. Pat. No. 3,058,909 which was used extensively in the oil and gas industry as a fracturing fluid. A fracturing process which comprised crosslinking, guar-containing compositions on-the-fly with borate ions was described by Free in U.S. Pat. No. 3,974,077. The borate-crosslinked systems require a basic pH (e.g. 8.5 to 10) for crosslinking to occur.
Other crosslinking agents were developed using certain transition metals. Chrisp described certain of these systems in U.S. Pat. No. 3,202,556 and U.S. Pat. No. 3,301,723. in U.S. Pat. No. 3,202,556 aqueous solutions of galactomannan gums were crosslinked at a pH of from about 6 to 13 with crosslinking agents selected from the group consisting of compounds of antimony and bismuth. In U.S. Pat. No. 3,301,723 Chrisp described the use of certain titanium, zirconium, and other transition metals as crosslinking agents for galactomannan gums at a pH also in the range from about 6 to about 13. In both Chrisp patents, a basic pH was used to prepare crosslinked materials having utility in the explosive industry.
Another patent which describes the use of titanium crosslinkers for solvatable polysaccharides was Tiner et al. (U.S. Pat. No. 3,888,312). The crosslinked gels formed by Tiner were said to be useful as fracturing fluids. The use of such crosslinked gels was alleged to overcome the high friction loss experienced during the pumping of many high viscosity fracturing fluids previously known. This observation corroborated the disclosure by Chrisp in U.S. Pat. No. 3,301,723 at column 10 that crosslinked gels formed by using titanium, chromium, iron and zirconium crosslinkers had a high surface tension (i.e. stickiness and tackiness are absent), ready workability and other desirable physical characteristics.
Chrisp and Tiner et al. each described titanium crosslinkers in which the "amine" portion of the crosslinker was a residue of triethanolamine. Chrisp in U.S. Pat. No. 3,301,723 at column 5, line 60 identified the crosslinker as titanium-triethanolamine chelates. Tiner et al. in U.S. Pat. No. 3,888,312, column 3 at lines 32-35 identifies the compound as bis(triethanolamine) bis(isopropyl)titanium (IV). Chrisp and Tiner at al. also disclosed a wide variety of other compounds in which the "anion" portion of the molecule was something quite different than the triethanolamine residue (e.g. chloride). Chrisp in U.S. Pat. No. 3,301,723 at column 4, lines 33-39 taught that the nature of the particular "anion" in the crosslinking agent was not critical but did have an influence on the solubility of the crosslinking compounds. Tiner et al. likewise taught that the crosslinking ability of their titanium crosslinking agent depended upon the presence of titanium in the +4 oxidation state and that the "anion" portion of the molecule could be carried. This broad teaching in Tiner et al. is once again analogous to the teaching in Chrisp.
As is well known, most such solvatable polysaccharides are typically crosslinkable in a basic aqueous medium (at a pH above 7) by a wide variety of organometallic compounds containing titanium or zirconium in a +4 oxidation (valence) state. These solvatable polysaccharides have a remarkable capacity to thicken aqueous liquids and thus to form gels. The crosslinked aqueous-base polysaccharide gels have been widely used as hydraulic fracture fluids for injection into subterranean formations to enhance the production of fluids therefrom. Hydraulic fracturizing is a term applied to a variety of techniques used to stimulate the production of oil, gas and other fluids from subterranean formations. In hydraulic fracturing, a suitable fracture fluid is introduced into a subterranean formation by way of a wellbore under conditions of flow rate and pressure which are at least sufficient to create and/or extend a fracture into a desired part of the formation. The fracture fluid normally carries with it a proppant (e.g. sand, bauxite, glass beads, etc.) which is forced into the fracture and keeps the broken formation from closing down upon itself once the pumping pressure is released.
The organometallic crosslinking agents containing titanium or zirconium in a +4 valence state likewise form a known class of compounds. A preferred class of zirconium crosslinking agent is disclosed in British Pat. No. 2,108,122 (Kucera), the disclosure of which is incorporated herein by reference. This class of crosslinking agents is prepared by reacting zirconium tetraalkoxides with alkanolamines under essentially anhydrous conditions. Other zirconium and titanium crosslinking agents are described, for example, in U.S. Pat. No. 3,888,312 (Tiner et al.), U.S. Pat. No. 3,301,723 (Chrisp), and U.S. Pat. No. 4,460,751 (Hanlon et al.), U.S. Pat. No. 4,477,360 (Almond), and Europe No. 92,755 (U.S. application Ser. No. 371,022, filed Apr. 22, 1982 by Rummo et al.), the disclosures of which are incorporated herein by reference. In all such prior teachings of organometallic crosslinking agents which included an alkanolamine ligand, the molar ratio of alkanolamine to zirconium or titanium was below 5.
A problem has been encountered in the use of these titanium and zirconium crosslinking agents. Under conditions of use, the organometallic crosslinkers are generally dissolved in water prior to blending into the polysaccharide solution. It has now been discovered that the functional effectiveness of such organometallic compounds as crosslinking agents deteriorates over a period of time in the presence of water. This problem is particularly acute when aqueous solutions of the crosslinkers must be held in storage over a period of several hours at high ambient temperature (e.g., above about 80.degree. F.).